: The investigators present a series of studies that link the soluble endothelial protein C receptor (EPCR), a member of the protein C anticoagulant pathway, with Wegener's granulomatosis, an autoimmune disease characterized by granulomatous inflammation and necrotizing vasculitis. They have observed that soluble EPCR can bind to activated neutrophils and this binding is mediated in large part by proteinase-3 (PR3), the target of the autoantibodies (cANCA) in Wegener's patients, and a beta2 integrin. EPCR binding to PR3 on cells or in purified systems is inhibited by cANCA. A monoclonal antibody to the beta2 chain of the leukocyte adhesion molecules (CD18) and an antibody to the unique alpha chain of Mac-1 (CD11b) also block binding of soluble EPCR to activated neutrophils. Finally, soluble EPCR can block tight adhesion of neutrophils to TNF alpha-activated endothelial cells. These observations elicit their working hypothesis that PR3 interaction with the beta2 integrin plays a role in leukocyte adhesion and soluble EPCR blocks this function by masking a key site on PR3. cANCA interferes with soluble EPCR binding to the PR3-beta2 integrin complex, thus preventing the negative regulatory function of EPCR. Their specific aims are to investigate the structure-function relationships of EPCR-PR3-beta2 integrin interactions by defining pair-wise interaction parameters and the influence of these complexes on neutrophil signaling and adhesion functions. The specific aims are to: 1) characterize the soluble EPCR-PR3 binding complex; 2) investigate the pair-wise interactions governing soluble EPCR-PR3-beta2 integrin complex formation; and 3) evaluate the influence of cANCA on the EPCR-PR3-beta2 integrin interaction with regard to neutrophil adhesion. Their studies focus on how soluble EPCR may influence leukocyte trafficking and the role of cANCA in modulating this process. Results from these studies will contribute to the identification of novel mechanisms of cANCA-mediated endothelial injury in vasculitis of Wegener's granulomatosis. In addition, the results will provide a foundation for future studies investigating potential therapeutic targets that modulate neutrophil-mediated endothelial injury, particularly for cANCA-mediated vasculitis.